Over the last century since the first successful demonstration of the Sagnac effect, there have been several attempts to develop an ultra-high-sensitivity device for continuous high resolution rotation sensing, such as the Michelson–Pearson–Gale interferometer in Clearing, Illinois. Apart from the application of a passive optical cavity,active ring laser gyroscopes with the gain medium inside the traveling wave ring cavity have also been used. This chapter briefly introduces the most important activities over the last century and puts them into context with our own large ring laser project.